Most current color CRTs employ a shadow mask separated by a designated distance from a phosphor-coated luminescent glass display screen. The shadow mask serves as a color selection electrode for selectively guiding electron beams emitted from electron guns onto designated phosphor coated portions on the luminescent screen formed on the inner surface of the display panel. The shadow mask is in the form of a thin metal sheet with a large number of electron beam passing apertures and is attached to a rigid peripheral frame. The frame is attached to and supported by an inner portion of the CRT's glass envelope.
The large number of small apertures in the shadow mask allow each of the three electron beams to be incident upon selected phosphor deposits on the inner surface of the display panel. Because apertures represent only approximately 20% of the total area of the shadow mask, approximately 80% of the energy of the electron beams is absorbed by the shadow mask and converted to heat energy as the electron beams impinge upon the shadow mask structure. This heat absorption by the shadow mask causes thermal deformation of the mask, which is commonly referred to as mask "doming." Doming of the shadow mask gives rise to a shift in electron beam landing position relative to the phosphor elements deposited on the display panel. This electron beam landing shift appears to the viewer as a degradation in video image brightness and color purity. Electron beam shift and the corresponding degradation in video image brightness and color purity increase with more closely spaced mask apertures i.e., finer aperture pitch, and flatter shadow masks, which are the trends in current color CRT design. A portion of the shadow mask connecting adjacent beam passing apertures is known as a "bridge" and serves as a mechanical support for the shadow mask. Each shadow mask bridge also serves as a barrier preventing at least a portion of the electron beam from penetrating the shadow mask and impinging on the CRT's display panel. Thus, the shadow mask bridges support and strengthen the shadow mask, but also contribute to thermal deformation of the mask and associated mask doming.
The present invention addresses the aforementioned limitations of the prior art by reducing shadow mask doming by reducing the number and sizes of the bridges extending between adjacent apertures in the mask.